1. Field of the Invention
The invention relates generally to vaccines used in veterinary applications and, more particularly, to a live, recombinant, attenuated vaccine for disease states that are caused by organisms that include capsule where the presence of the capsule is required for virulence but not immunoprotection. The invention has specific application to a recombinantly produced vaccine that has been engineered such that it lacks capsule.
2. Description of the Prior Art
Vaccines are preparations used to prevent specific diseases in animals and humans by inducing immunity. This is accomplished by exposing a patient to an antigen for a particular disease which, in turn, causes the immune system of the patient to produce large quantities of antibody. The presence of the antibody in the patient's blood protects the patient from a later attack by the disease causing agent. Vaccines may either be composed of subunits of the agent, or the live or killed agent itself. For example, poliomyelitis, commonly referred to as "polio", is typically prevented by either administering a live, attenuated oral poliovirus vaccine, which is common practice for treating children, or by administering a killed or inactivated poliovirus vaccine, which is the usual practice for treating adults since they are generally at higher risk for contracting polio from the live vaccine. If a live vaccine is to be used, its virulence must be attenuated in some way; otherwise the virus in the vaccine will cause the disease it is intended to protect against.
A number of diseases are caused by encapsulated bacteria wherein the capsule, which is the gum-like layer of polysacharide or polypeptide exterior to the cell wall of these bacteria, is required for pathogenesis. Swine pleuropneumonia is one example, and virulence factors for Actinobacillus pleuropneumoniae, the bacterium which causes the disease, include capsular polysaccharide, endotoxin, and protein exotoxins. Swine pleuropneumonia is one of the major respiratory diseases affecting swine production throughout the world, and accounts for millions of dollars in annual losses to the industry in the United States alone.
U.S. Pat. No. 5, 429,818 to Inzana, which is herein incorporated by reference, discloses that non-encapsulated mutants of Actinobacillus pleuropneumoniae are avirulent and capable of providing excellent protection against subsequent exposure to the virulent bacteria. The non-capsulated mutants described in Inzana were prepared by ethylmethanesulfunate mutagenesis. However, such procedures have the disadvantages that some spontaneous or chemically induced mutants may not be stable, and the nature of the mutation(s) is (are) unknown.